Implantable stimulation devices are devices that generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system. However, the present invention may find applicability in any implantable medical device system.
As shown in FIG. 1, a SCS system typically includes an Implantable Pulse Generator (IPG) 10, which includes a biocompatible device case 12 formed of titanium for example. The case 12 typically holds the circuitry and battery 26 necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 10 is coupled to electrodes 16 via one or more electrode leads (two such leads 19 and 20 are shown), such that the electrodes 16 form an electrode array 22. The electrodes 16 are carried on a flexible body 24, which also houses the individual signal wires 27 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead 19, labeled E1-E8, and eight electrodes on lead 20, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. The leads 19 and 20 couple to the IPG 100 using lead connectors 28, which are fixed in a header material 30 comprising an epoxy for example. Two coils (antennas) are generally present in the IPG 100: a telemetry coil 13 used to transmit/receive data to/from an external controller; and a charging coil 18 (inside case 12) for charging or recharging the IPG's battery 14 using an external charger.
FIG. 2 shows an Application Specific Integrated Circuit (ASIC) 300 used in an IPG such as IPG 10, which is disclosed in FIG. 4B of U.S. Patent Application Publication 2012/0095519, which is incorporated herein by reference. The same element numerals as disclosed in the '519 Publication are used in current FIG. 2, and the reader can refer to that publication for a more thorough description of ASIC 300. Generally speaking, the ASIC 300 attempts to integrate as much of the functionality in the IPG 10 as possible in a single Integrated Circuit (IC). Thus, the ASIC 300 includes different modules for controlling operation of various functions within the IPG, such as: charging/protection circuitry 64 for receiving energy from the charging coil 18 and using that energy to charge the IPG's battery 26; telemetry circuitry 62 coupled to the telemetry coil 13 to send and receive data externally from the IPG 10; stimulation circuitry 175 for forming the stimulation pulses to be provided to the electrodes, etc. It is not necessary here to describe the full functionality of ASIC 300.
What is important to note is that the ASIC 300 requires a clock signal to function. Typically, and as shown in FIG. 2, the clock signal was provided to the ASIC 300 by a crystal oscillator 340 at a clock input pin (CLKIN) on the ASIC. As is well known, a crystal 340 is a piezoelectric material that when biased resonates to create a signal with a very precise frequency. The frequency produced by the crystal 340 has a low temperature coefficient—meaning that it does not change significantly with temperature—and has a low frequency drift over time. Thus, a crystal 340 provides a stable clock signal, CLK, of a predictable frequency to the ASIC 300.
The inventors realize that use of a crystal 340 to provide a clock signal has disadvantages. As a discrete electro-mechanical device, the crystal 340 cannot be fully integrated into the ASIC 300. This is unfortunate, because the crystal 340 takes up some room on the printed circuit board (PCB; not shown) within the IPG, which room is ever-diminishing as IPGs are made smaller. A crystal 340 is also subject to mechanical damage, and can be susceptible to circuit parasitics present on the PCB, which can affect start-up time and performance. A crystal 340 is also slow to start, i.e., it takes some time (e.g., 15 seconds) to start producing its predictable frequency, which is undesirable in an application as critical as a medical implant. Moreover, the crystal 340, being a discrete component, adds cost to the manufacture of the IPG 10.